Technical Field
The present invention relates to a control rod drive mechanism and more particularly relates to a control rod drive mechanism suitably applicable to a boiling water reactor.
Background Art
A boiling water reactor includes a reactor pressure vessel, and a core shroud, an upper lattice plate, a core support plate, a separator, and a steam dryer installed in the reactor pressure vessel. A core in which a plurality of fuel assemblies are loaded is surrounded by the core shroud. A core support plate is disposed below the core and is attached to the core shroud. An upper lattice plate is disposed above the core and is attached to the core shroud. The core is disposed between the core support plate and the upper lattice plate. The separator is disposed above the upper lattice plate and the steam dryer is disposed above the separator.
A lower end of each fuel assembly loaded in the core is supported by a plurality of fuel supports installed on the core support plate. The upper end of each fuel assembly is supported by the upper lattice plate.
A plurality of control rods withdrawing from and inserting into among the fuel assemblies in order to control the reactor power are disposed in the reactor pressure vessel. These control rods are respectively connected to control rod drive mechanisms. Each control rod drive mechanism is stored separately in a control rod drive mechanism housing installed at the bottom of the reactor pressure vessel. Cooling water serving as moderator and coolant is filled in the reactor pressure vessel.
An example of the control rod drive mechanism used in the boiling water reactor is described in Japanese Patent Laid-Open No. 10(1998)-132977. The control rod drive mechanism includes an outer tube, a guide tube, a piston tube, a ball screw, a ball nut, a spool piece, an inner magnet coupling, an outer magnet coupling, and a motor. A flange of the outer tube is attached to a flange of the spool piece by a plurality of bolts, as described in FIG. 1 of Japanese Patent Laid-Open No. 10(1998)-132977. The guide tube is disposed in the outer tube and a lower end of the guide tube is supported by the outer tube. The piston tube is disposed in the guide tube. The ball screw is inserted in a hollow portion formed around a central axis of the piston tube. The ball nut mates with the ball screw and a lower end of the piston tube is put on a top face of the ball nut. An upper end portion of the piston tube is coupled removably to a lower end portion of the control rod. A back sheet is provided in the lower end portion of the ball screw. The back sheet is disposed opposite to a top face of the back sheet receiving portion installed inside the outer tube in the lower end portion of the outer tube. In the normal state of each control rod drive mechanism, a gap is formed between an undersurface of the back sheet and the top face of the back sheet receiving portion, and the undersurface of the back sheet is not in contact with the top face of the back sheet receiving portion.
An inner magnet coupling to which a rotary shaft is attached is disposed inside the spool piece attached to the outer tube. The rotary shaft is connected removably to the back sheet.
A cylindrical outer magnet coupling is disposed so as to surround a lower end portion of the spool piece. The outer magnet coupling is connected to the rotary shaft of the motor attached to the spool piece by the support member. An outer magnet installed on the inner surface of the outer magnet coupling is opposite to an inner magnet installed on the outer surface of an inner magnet bearing and a side wall of the spool piece is disposed between the inner magnet and the outer magnet.
The outer tube of the control rod drive mechanism is inserted into the control rod drive mechanism housing installed on a bottom head of the reactor pressure vessel and the flange of the outer tube and the flange of the spool piece which are connected by a bolt are attached to the flange of the control rod drive mechanism housing by other a plurality of bolts (refer to FIG. 1 of Japanese Patent Laid-Open No. 10(1998)-132977).
The control rod drive mechanism shown in FIG. 1 of Japanese Patent Laid-Open No. 10(1998)-132977 does not require a rotary shaft for connecting a inner magnet coupling and an outer magnet coupling and spool piece forms a pressure boundary and prevents cooling water in the reactor pressure vessel from leaking to the outside. Particularly, an O-ring disposed between the flange of the control rod drive mechanism housing and the flange of the spool piece and an O-ring arranged between the flange of the outer tube and the flange of the spool piece prevent the cooling water from leaking to the outside.
The rotary power of the motor is transmitted to the inner magnet coupling by the outer magnet coupling to rotate the ball screw. The ball nut moves up and down by the rotation of the ball screw and by doing this, the control rod is inserted into the core or is withdrawn from the core.
In the control rod drive mechanism shown in FIG. 1 of Japanese Patent Laid-Open No. 10(1998)-132977, as mentioned above, the flange of the outer tube and the flange of the spool piece which are combined by a bolt are attached to the flange of the control rod drive mechanism housing by other a plurality of bolts. On the other hand, in the control rod drive mechanism shown in FIG. 1 of Japanese Patent Laid-Open No. 10(1998)-132977, as shown in FIG. 5 of Japanese Patent Laid-Open No. 10(1998)-132977, in the state that the flange of the outer tube is attached to the flange of the control rod drive mechanism housing by a bolt, the flange of the spool piece may be attached to the flange of the control rod drive mechanism housing by another bolt. In such a structure, an O-ring is disposed between the flange of the control rod drive mechanism housing and the flange of the outer tube and furthermore, an O-ring is disposed between the outer surface of the outer tube and the inner surface of the spool piece.
Also in Japanese Patent Laid-Open No. 8(1996)-82690, a control rod drive mechanism having the same structure as that of the aforementioned control rod drive mechanism described in Japanese Patent Laid-Open No. 10(1998)-132977 is described.
Also in the control rod drive mechanism described in Japanese Patent Laid-Open No. 60(1985)-47987, the ball screw is rotated by the motor and the ball nut mating with the ball screw moves up and down. As a result, the control rod is inserted into the core or is withdrawn from the core. In the control rod drive mechanism, the flange existing in the upper end portion of the cylindrical spool piece is attached to the flange of the control rod drive mechanism housing and a cylinder body is attached to the flange in the lower end portion of the spool piece. The lower end of the guide tube of the control rod drive mechanism is put on the top face of the cylinder body and the guide tube is supported by the cylinder body.